An SHG (second harmonic generation) laser light source has been proposed (see Japanese Patent Application Laid-Open No. 2002-043683). FIG. 8 shows its structure. As shown in this drawing, provided are a DBR laser 1010 which includes a DBR part 1013, a phase part 1012, and a gain part 1011, a SHG device 1020, and a control part 1030. The control part 1030 changes a current flowing through the DBR part 1013 and the phase part 1012 at a predetermined change current ratio to modulate an intensity of SHG light. The control part 1030 modulates the intensity of SHG light based on a result obtained by detection of a photo detector that receives second harmonic light from the SHG device 1020.
A method of modulating a current flowing through the gain part 1011 based on a PWM (pulse width modulation) signal and controlling an oscillation time of the laser to thereby express gradation levels has been generally used for a laser beam printer and the like.
In Japanese Patent Application Laid-Open No. 2002-043683, a method of changing a current flowing through the DBR part 1013 and the phase part 1012 at the predetermined change current ratio to modulate an intensity of SHG light is used and effective to express binary values of ON-OFF in a recording application such as an optical disk. However, the inventors of the present invention found a problem in that such a technique requires a high-precision analog circuit in order to realize high-resolution gradation level expression and the linearity of gradation expression is impaired in the case where a peak point of conversion efficiency of the SHG light is deviated due to a change in environmental temperature, or the like.
In Japanese Patent Application Laid-Open No. 2002-043683, shown is an example in which a desirable intensity of the SHG light is obtained by injecting a current based on an arbitrary pattern of the PWM signal to the gain part 1011. In this case, there is a problem in that a temperature of the DBR laser 1010 and a temperature of the light wavelength conversion device 1020 depend on the pattern of the PWM signal. As a result, an oscillation wavelength of the DBR laser 1010 and a phase matching wavelength of the light wavelength conversion device 1020 depend on the pattern of the PWM signal. Therefore, the intensity of the SHG light does not become constant.